This invention relates to control devices for a heating or air conditioning system, and more particularly to control devices for cycling or pulsating the heating or cooling units to obtain more efficient operation of the system.
In automatic heating systems, it is customary to provide a heating control system which comprises a thermally responsive device, such as bimetallic thermostat, in the space to be heated, and associated with such thermostat, means for operating the heater controls. The heater controls are ordinarily removed from the space to be heated, and the bimetallic thermostat, which is a relatively delicate instrument with low power, ordinarily operates the remotely located heater controls via an electrical connection.
The bimetallic thermostat is generally connected in a series arrangement with a low-voltage power source and an actuating device for operating the heater controls. The bimetallic thermostat generally consists of a switch actuated by a bimetallic metal. When the space to be heated is at or above the desired temperature setting (i.e., the temperature set on the bimetallic thermostat), the bimetallic thermostat is open rendering the heater non-operative. When the temperature in the space to be heated falls below the desired temperature, the bimetallic thermostat closes activating the heater control, which in turn activates the heater, thereby supplying heat to the space to be heated.
In order to maintain an even temperature in the space to be heated, it is desirable that the bimetallic thermostat respond quickly to temperature changes. However, because of the many thermal characteristics of the heating system, including the thermal time constant of the thermostat, the thermostats of the present art do not respond quickly enough, thereby causing room temperatures to overshoot or go higher than the setting of the bimetallic thermostat before the bimetallic thermostat opens deactivating the heater controls.
The bimetallic thermostats of the present art are two-position control devices, i.e., the control causes the burner to be full-on or full-off. One of the difficulties with a two-position control is that frequently the control keeps the heater activated for too long a time. Consider a house which utilizes hot-water radiation heat. When the space to be heated is below the bimetallic thermostat setting, the heater is activated (by the closure of the bimetallic thermostat), heating the water which will rise by convection to displace the colder water in the radiators. The bimetallic thermostat will keep the heater going until the bimetallic thermostat senses the desired temperature setting. At this point, the bimetallic thermostat opens causing the heater to deactivate, i.e., shut-down. However, the thermal mass residual in the heating system, which has been activated full-on up to the point in which the bimetallic thermostat opens, is enough to cause the heat output into the space to be heated to continue beyond the setting of the bimetallic thermostat, and some temperature overshoot of the space to be heated will occur. This is easily understood when the mass of the radiators is considered, how warm they get, and how long it takes for the radiators to dissipate their heat once the heater is shut down.
To compensate for the temperature overshoot, two-position control devices including the bimetallic thermostat of the present art, frequently incorporate an anticipator which, upon the closure of the bimetallic thermostat contacts, applies false heat to the bimetallic thermostat causing the heater to shut-down permaturely, i.e., the temperature in the space to be heated has not reached the desired temperature setting. This premature shut-down allows the heating system to take advantage of the heating system's thermal characteristics (i.e., the thermal time constants) and permit the temperature in the space to be heated to rise more gradually to the thermostat temperature setting before re-activating the heater. Present art heat anticipators are made of resistance type material that produces heat in accordance with the current drawn through them. Heat anticipators of the present art are adjustable and are normally set to correspond with the current rating of the main gas valve, thereby essentially ignoring the thermal characteristics of the space to be heated.
The phenomenon of thermal mass or thermal time constants can be explained by the act of attempting to boil water utilizing an electric stove. The burner is turned on and the electric energy supplied to the burner is transferred to the water. When the water boils, the pot is removed from the burner, the water temperature having reached the desired temperature, and the burner is turned off. However, it will be observed that the burner is still glowing cherry red and the residual heat in the burner element will now be transferred to the room and not be utilized for heating the water. If the burner had been turned off slightly before the water started to boil, the burner would have remained cherry red for a period of time and, during this period of time, the residual heat in the burner would have gone into heating the water causing the water to boil as before but results in a savings of electrical energy supplied to the burner.
The present invention provides in the heating control system, a novel operation of the heater control by causing the heater to cycle on and off until the temperature in the space to be heated has reached the desired temperature setting. The incremental additions of heat prevent overshoots in the space temperature by taking advantage of the thermal characteristics of the heating system thus resulting in a fuel savings. No adjustment akin to that of present anticipators is necessary in the present invention. By providing an adjustment of the cycle-on and cycle-off time, the present invention is directed towards maximizing the utilization of the thermal characteristics of the heating system.